FIGS. 1(a) and 1(b) illustrate a known method of manufacturing shells for wind turbine blades. Referring to FIG. 1(a), a first layer 1 of dry glass fibre fabric is laid on the surface of a lower half-mould 2, and this will form an outer skin 1 of a half-shell of a wind turbine blade. Two stacks 3 of pultruded fibrous composite strips are introduced into channels formed between layers 4 of structural foam, as indicated by the arrows in the drawing. The stacks form the spar caps of the wind turbine blade.
Referring to FIG. 1(b), strips of pre-cured glass fibre epoxy resin composite 5 are then placed along the surfaces of the stacks 3 so that they cover not only the stacks 3 but also the margins of the foam layers 4. In this way, the interfacial regions of the stacks 3 and the foam layers 4 are protected by the glass fibre strips 5. A second layer 6 of dry glass fibre fabric is then placed over the surfaces of the strips 5 and this second glass fibre layer 6 will form an inner skin 6 of a lower half-shell of the wind turbine blade.
For the sake of conciseness, the pre-cured glass fibre epoxy resin composite material is referred to below simply as glass fibre.
An air-tight sealing layer 7 in the form of a vacuum bag is then attached to the half-mould 2 so as form an evacuation chamber encapsulating all of the components, and the chamber is then evacuated using a vacuum pump 8. This causes the second glass fibre layer 6 to press on the upper surfaces of the glass fibre strips 5 and the foam layers 4.
With the pump 8 still energised, a supply 9 of liquid resin is connected to the chamber so as to infuse the components and the interstitial spaces therebetween. The half-mould 2 is then heated so as to cure the composite materials. Resin film may also be placed between the pultruded fibrous composite strips when the stacks 3 are placed in the mould to aid the adhesion between the strips.
The sealing layer 7 is then removed, and a respective elongate web (not shown) is positioned on the surface of the inner skin 6 at a position above each of the stacks 3.
A corresponding process is applied to the components of an upper half of the shell within a further half-mould which is substantially identical to the half-mould 2. The pump 8 continues to operate during a subsequent moulding operation in which the mould halves are heated so as to cure the resin, although during the curing process the extent of de-pressurisation may be lowered. The upper half-mould is then pivoted into position above the half-mould 2, and the two half-moulds are joined together.
During manufacture, the tolerances of the thicknesses of the abutting stacks 3 and the foam layers 4 are such that it is not always possible to match the thicknesses. This can result in step-shaped discontinuities along the inner circumferential surfaces of the shells. When covered with the second glass fibre layer 6, these discontinuities can lead to undesirable cracking of both the glass fibre strips 5 and the second glass fibre layer 6 when the wind turbine blade is subjected to fatigue load.
This problem is illustrated in FIGS. 2(a) and 2(b), in which, for the sake of clarity, the second glass fibre layer 6, the outer skin 1 and the half-mould 2 are omitted and the components are not drawn to scale. FIG. 2(a) illustrates the case where the thickness of the foam layers 4 is less than that of the stacks 3, and, conversely, FIG. 2(b) illustrates the case where the thickness of the foam layers 4 is greater than that of the stacks 3.
This difference in the thickness of the stacks 3 and the foam layers 4 has been found to give rise to undesirable stresses within the glass fibre strips 5 along the lines 10, which can result in fracture of the glass fibre strips 5, with resulting instability in the wind turbine blade. Corresponding stresses and fractures could also result in the second glass fibre layer 6.
It would therefore be desirable to provide a method of making such wind turbine blades which overcomes, or at least mitigates, this problem.